Slimy mats of bacteria called biofilms may be the most liquid-repellent materials in nature, researchers have discovered.

“There are a few man-made materials that can perform better, and they have to be made in clean rooms. They’re incredibly expensive and brittle,” said materials scientist Alexander Epstein of Harvard University, co-author of the new study. “Making biofilm is as easy as growing bacteria.”
The goo secreted by Bacillus subtilis bacteria not only deflects water like a lotus leaf, but also repels concentrated alcohol, acetone and even vaporized liquid, according to a study published Jan. 18 in Proceedings of the National Academy of Sciences.

Biofilms are communities of bacteria that stick together using a mixture of sugars and proteins called the extracellular matrix, which takes on a wrinkled form under powerful microscopes (see image below). Since the discovery of bacteria in the late 1600s, most research has covered individual cells. It’s only since the 1990s that scientists have begun to understand the pervasiveness and importance of biofilms.

“We’re realizing more and more that almost all bacteria in nature are found as biofilms,” Epstein said. “It offers a lot of advantages for them, including better protection and increased chances they’ll stick to sources of food. It’s crazy stuff.”

While trying to map the internal structure of B. subtilis biofilm using a vaporized radioactive tracer that would show up in x-ray photos, the researchers found it couldn’t get inside. They were frustrated at first, but Epstein said his team soon realized the significance of the stubborn biofilm.
“We started to put different liquids on it. Concentrated alcohols just [beaded up],” Epstein said. For comparison, the researchers also tried a non-stick Teflon surface. “We found the biofilms substantially superior to Teflon,” he said.

To analyze resistance to wetting, Epstein and his team measured the angle between droplets of liquid and the materials’ surfaces (right). At 10 percent alcohol, similar to wine’s concentration, Teflon started getting wet. Biofilms, however, balled up all concentrations of alcohol up to 80 percent (similar to Everclear). Liquids wetted biofilms only after sitting on them for minutes or hours.
Epstein isn’t suggesting we grow biofilms for frying pans or rain jackets, but thinks the research will inspire practical new applications. His team ultimately hopes to resolve biofilm’s molecular structure to develop new materials.
“We don’t fully understand it, but it’s broadly true that you need both protein and sugars for this repelling property,” Epstein wrote in an e-mail to Wired.com. “Sugars absolutely have to be there, but we don’t know why. We’re doing some followup research on that now,” he said.

Images: Courtesy of PNAS/Alexander Epstein et al. 1) A bead of 50 percent ethanol rests on a biolfilm of B. subtilis bacteria sliced from a Petri dish. 2) Wetness of a surface is determined by the contact angle of liquid droplets, and increases sharply when a droplet’s inner contact angle is less than 90 degrees. 3) A scanning electron micrograph of a mutant B. subtilis strain’s extracellular matrix, or ECM.

• By Dave Mosher
• January 31, 2011